1. Field of the Invention
The invention relates to emergency lighting, and particularly to fluorescent lighting wherein a fluorescent lamp is connected to a source of electrical energy other than normal AC line current in the event that the normal AC current fails.
Emergency lighting is required in commercial, industrial, and institutional buildings just as fire extinguishers, smoke alarms and other safety equipment. Three types of emergency lighting are common in such installations: unit equipment, engine generators and central battery systems. Unit equipment falls into two principle types: fluorescent and incandescent.
The fluorescent emergency units are customarily combined with and within a conventional fluorescent lighting unit by merely adding the emergency ballast consisting of a battery, a battery charger, inverter and sensing circuitry adjacent the standard fluorescent AC ballast. The sensing circuit of the emergency ballast observes the interruption of normal AC power to the lamp unit and immediately switches on the emergency ballast to power individual lamp(s) or the light fixture for the required period which, under most state safety codes, is a period of at least ninety (90) minutes, a standard called out in the National Electrical Code, NFPA Article 70, and NFPA Article 101 Light Safety Code.
2. General Background of the Invention
U.S. Pat. No. 5,004,953 entitled Emergency Lighting Ballast for Compact Fluorescent Lamps with Integral Starters, assigned to the assignee of the present invention is illustrative of the general fluorescent type of emergency lighting with a ballast and also illustrates a compact fluorescent lamp. It is common in the installation of emergency fluorescent lighting that an emergency ballast is added to a conventional fluorescent fixture or provided integrally in a fixture having internal regular and emergency ballast installed. When main AC power fails, voltage sensing circuitry instantly connects DC current from a battery (in the emergency ballast) to an inverter which produces high frequency, high voltage power to illuminate the emergency fluorescent lamp(s) for the required period.
The present invention is directed to fluorescent lighting fixtures which are likely to be exposed to extreme low temperatures wherein the battery which supplies the power for lighting when main AC power fails is apt to freeze. Until now, emergency lighting has been utilized in such as commercial an governmental buildings wherein it is unlikely that power will be out for such a long period of time that the batteries for providing the back-up power will freeze. The state of the art units typically are recommended for temperature ranges of about 0° C. to about 50° C. Unit contained emergency fluorescent lighting is now so reliable and common that it is being considered for such as smaller out buildings, garages, outside staircases and warehouses where the building (or fluorescent unit) is unheated, or if subjected to prolonged power outage during winter, the internal environment may cool to 0° C. or lower. After a prolonged time at below freezing temperatures, the battery internal to the emergency ballast is likely to freeze. The problem arises when normal AC power is restored and the battery charging circuit of the unit attempts to recharge the battery. With the inclusion of the present invention, emergency fluorescent lighting may be extended to an operating range of about −20° C. to about 50° C.
Emergency fluorescent lighting has now advanced to the state of art wherein compact lamps as well as conventional (large) fluorescent lamps are finding application in extreme low temperature sites, particularly those where operation of the lamp, as well as the emergency ballast, are disadvantageously effected. Compact Fluorescent lamps present special issues when operated in low power ranges, and operation of such lamps in colder than normal settings causes permanent and rapid deterioration of the lamp. Among the observed problems are cathode instability including sputtering and the deposit of a pink haze on the glass envelope of the lamp.
Compact fluorescent lamps are designed to be operated on a-c current and at rated power, i.e., about 42 Watts. In emergency operation in conjunction with an emergency ballast, all fluorescent lamps are operated at power levels that are considerably below rated, and in the instance of the compact lamp, anywhere between about 7 to about 12 Watts, depending upon the particular lamp. Likewise, the emergency operating current is supplied by a battery in the ballast, and is a variety of a pulsating d-c current except compact lamps operate more efficiently with AC current. It has become evident that the degree of heating generated by the normal AC current at a level of around 7–12 Watts of output is nowhere close to the level in emergency mode for generating sufficient heat energy to sustain a satisfactory continuing operation for a compact lamp in a severely low temperature environment.
It is likewise known that rechargeable batteries such as the nickel-cadmium type typically used in fluorescent emergency lighting systems have difficulties in charging when frozen. Charging may be erratic, and at times at rates much higher than the rate recommended by manufacturers for safe operation. There is also a danger that during the charging process that volatile gasses, particularly hydrogen, may build-up and pose the threat of explosion. The present invention provides a heating mechanism for the rechargeable battery which keeps it at a suitable charging temperature during normal operation whether the installation site building is heated or not, enabling the use of the unit in unheated or lesser heated facilities. Further, the heater circuit addition includes means for connecting the charging circuit to a dummy load while the battery comes up to suitable temperature for charging, while keeping the appropriate relays connected so that upon the resumption of AC power, the normal fluorescent lighting may be powered by the AC main power. Once the battery temperature is sensed to be at a sufficient level for safe charging, the charging circuit is switched to the battery from the dummy load so as to fully recharge the battery. In conjunction with these improvements useful for standard or large sized fluorescent lamps, the present invention related to compact fluorescent lamps includes providing an additional cathode heating current to at least one of the filaments of the compact lamp, which filament is provided with a current (a-c, preferably or d-c) at a level of about 3 to 4 volts. The heating current may be supplied from a separate battery or the battery supplying the current to illuminate the fluorescent lamp during emergency operation
Those skilled in the art will recognize that alternative approaches to the preferred embodiments of the present invention disclosed herein may be made without departing from the scope of the present invention.